1. Field of the Invention
The present invention relates to a method for producing an endless track link for vehicles such as a power-shovel, a bulldozer, and the like. More particularly the present invention pertains to a method for producing an endless track link wherein a roller contact surface of the link is not subjected to separate high-frequency induction-hardening, tempering, and preliminary machining steps.
2. Description of the Related Art
As illustrated in FIG. 5, a conventional method for producing a vehicular endless track link involves sequentially performing the steps of forging a link material, quench-hardening the link material while the link material is at an elevated temperature (the elevated temperature being realized by either utilizing the residual heat of the forging step, by reheating the link material, or by a combination thereof), tempering the link material, machining the end surfaces of the link material, high-frequency induction-hardening a roller contact surface of the link material, tempering the roller contact surface, preliminarily machining a pin hole and a bushing hole, machine finishing the pin hole and the bushing hole, and machining nut seat surfaces of the link. As described above, the conventional method requires that separate induction-hardening and tempering steps be preformed specifically on the roller contact surface.
However, the conventional method illustrated in FIG. 5 and described above possesses several disadvantages.
First, because the roller contact surface is not subjected to induction-hardening or tempering until after the entire link has been hardened and tempered, the conventional method is characterized by a high thermal energy cost.
Second, because the entire link is tempered at a high temperature to allow for its machining after it has been quench-hardened, the hardness obtained from the quench-hardening step is not maintained in the resulting track link.
Two methods for producing a vehicular endless track wherein high-frequency induction-hardening is omitted on the roller contact surface have been proposed to overcome these disadvantages.
The first proposed method is disclosed in Japanese Patent Publication No. HEI 5-9488. According to this conventional method, during heat treatment the metallic crystal structure of the roller contact surface of the link is converted to martensite by rapidly cooling the roller contact surface within oil. The metallic crystal structure of a remaining portion of the link is converted to bainite by cooling the remaining portion in wind, so that high-frequency induction-hardening of the roller contact surface is unnecessary, while the remaining portion is relatively soft and can be machined.
According to the second method disclosed in Japanese Patent Publication No. SHO 57-51583, the portion of a link to be machined is tempered at a high temperature by induction-heating. Tempering a portion of the link is essential because if the entire link is hardened (i.e., if no portion is subjected to induction-heating), the link cannot be machined. In both of the above-mentioned methods, a portion of the link to be machined is heat-treated to be softer than the roller contact surface.
Although the method of Publication No. HEI 5-9488 has been found to overcome the above-described problem of high thermal energy cost associated with the convention method, it does not adequately solve the above-described second problem. Further, the productivity of the method is poor because the cooling method during hardening is complicated.
With respect to the method disclosed in Publication No. SHO 57-51583, it too resolves the above-described problem of high thermal energy cost. However, this method also does not overcome the above-described second problem. Further, because the machined portion is tempered at a high temperature, there are no large advantages with respect to cost and quality.